A CGI script is invoked by an HTTP server, usually to process user input
submitted through an HTML <FORM> or <ISINDEX> element.

Most often, CGI scripts live in the server’s special cgi-bin directory.
The HTTP server places all sorts of information about the request (such as the
client’s hostname, the requested URL, the query string, and lots of other
goodies) in the script’s shell environment, executes the script, and sends the
script’s output back to the client.

The script’s input is connected to the client too, and sometimes the form data
is read this way; at other times the form data is passed via the “query string”
part of the URL. This module is intended to take care of the different cases
and provide a simpler interface to the Python script. It also provides a number
of utilities that help in debugging scripts, and the latest addition is support
for file uploads from a form (if your browser supports it).

The output of a CGI script should consist of two sections, separated by a blank
line. The first section contains a number of headers, telling the client what
kind of data is following. Python code to generate a minimal header section
looks like this:

print("Content-Type: text/html")# HTML is followingprint()# blank line, end of headers

The second section is usually HTML, which allows the client software to display
nicely formatted text with header, in-line images, etc. Here’s Python code that
prints a simple piece of HTML:

This activates a special exception handler that will display detailed reports in
the Web browser if any errors occur. If you’d rather not show the guts of your
program to users of your script, you can have the reports saved to files
instead, with code like this:

importcgitbcgitb.enable(display=0,logdir="/path/to/logdir")

It’s very helpful to use this feature during script development. The reports
produced by cgitb provide information that can save you a lot of time in
tracking down bugs. You can always remove the cgitb line later when you
have tested your script and are confident that it works correctly.

To get at submitted form data, use the FieldStorage class. If the form
contains non-ASCII characters, use the encoding keyword parameter set to the
value of the encoding defined for the document. It is usually contained in the
META tag in the HEAD section of the HTML document or by the
Content-Type header). This reads the form contents from the
standard input or the environment (depending on the value of various
environment variables set according to the CGI standard). Since it may consume
standard input, it should be instantiated only once.

The FieldStorage instance can be indexed like a Python dictionary.
It allows membership testing with the in operator, and also supports
the standard dictionary method keys() and the built-in function
len(). Form fields containing empty strings are ignored and do not appear
in the dictionary; to keep such values, provide a true value for the optional
keep_blank_values keyword parameter when creating the FieldStorage
instance.

For instance, the following code (which assumes that the
Content-Type header and blank line have already been printed)
checks that the fields name and addr are both set to a non-empty
string:

form=cgi.FieldStorage()if"name"notinformor"addr"notinform:print("<H1>Error</H1>")print("Please fill in the name and addr fields.")returnprint("<p>name:",form["name"].value)print("<p>addr:",form["addr"].value)...furtherformprocessinghere...

Here the fields, accessed through form[key], are themselves instances of
FieldStorage (or MiniFieldStorage, depending on the form
encoding). The value attribute of the instance yields
the string value of the field. The getvalue() method
returns this string value directly; it also accepts an optional second argument
as a default to return if the requested key is not present.

If the submitted form data contains more than one field with the same name, the
object retrieved by form[key] is not a FieldStorage or
MiniFieldStorage instance but a list of such instances. Similarly, in
this situation, form.getvalue(key) would return a list of strings. If you
expect this possibility (when your HTML form contains multiple fields with the
same name), use the getlist() method, which always returns
a list of values (so that you do not need to special-case the single item
case). For example, this code concatenates any number of username fields,
separated by commas:

value=form.getlist("username")usernames=",".join(value)

If a field represents an uploaded file, accessing the value via the
value attribute or the getvalue()
method reads the entire file in memory as bytes. This may not be what you
want. You can test for an uploaded file by testing either the
filename attribute or the file
attribute. You can then read the data at leisure from the file
attribute (the read() and readline()
methods will return bytes):

If an error is encountered when obtaining the contents of an uploaded file
(for example, when the user interrupts the form submission by clicking on
a Back or Cancel button) the done attribute of the
object for the field will be set to the value -1.

The file upload draft standard entertains the possibility of uploading multiple
files from one field (using a recursive multipart/* encoding).
When this occurs, the item will be a dictionary-like FieldStorage item.
This can be determined by testing its type attribute, which should be
multipart/form-data (or perhaps another MIME type matching
multipart/*). In this case, it can be iterated over recursively
just like the top-level form object.

When a form is submitted in the “old” format (as the query string or as a single
data part of type application/x-www-form-urlencoded), the items will
actually be instances of the class MiniFieldStorage. In this case, the
list, file, and filename attributes are always None.

A form submitted via POST that also has a query string will contain both
FieldStorage and MiniFieldStorage items.

The previous section explains how to read CGI form data using the
FieldStorage class. This section describes a higher level interface
which was added to this class to allow one to do it in a more readable and
intuitive way. The interface doesn’t make the techniques described in previous
sections obsolete — they are still useful to process file uploads efficiently,
for example.

The interface consists of two simple methods. Using the methods you can process
form data in a generic way, without the need to worry whether only one or more
values were posted under one name.

In the previous section, you learned to write following code anytime you
expected a user to post more than one value under one name:

item=form.getvalue("item")ifisinstance(item,list):# The user is requesting more than one item.else:# The user is requesting only one item.

This situation is common for example when a form contains a group of multiple
checkboxes with the same name:

In most situations, however, there’s only one form control with a particular
name in a form and then you expect and need only one value associated with this
name. So you write a script containing for example this code:

user=form.getvalue("user").upper()

The problem with the code is that you should never expect that a client will
provide valid input to your scripts. For example, if a curious user appends
another user=foo pair to the query string, then the script would crash,
because in this situation the getvalue("user") method call returns a list
instead of a string. Calling the upper() method on a list is not valid
(since lists do not have a method of this name) and results in an
AttributeError exception.

Therefore, the appropriate way to read form data values was to always use the
code which checks whether the obtained value is a single value or a list of
values. That’s annoying and leads to less readable scripts.

A more convenient approach is to use the methods getfirst()
and getlist() provided by this higher level interface.

This method always returns only one value associated with form field name.
The method returns only the first value in case that more values were posted
under such name. Please note that the order in which the values are received
may vary from browser to browser and should not be counted on. [1] If no such
form field or value exists then the method returns the value specified by the
optional parameter default. This parameter defaults to None if not
specified.

This method always returns a list of values associated with form field name.
The method returns an empty list if no such form field or value exists for
name. It returns a list consisting of one item if only one such value exists.

Using these methods you can write nice compact code:

importcgiform=cgi.FieldStorage()user=form.getfirst("user","").upper()# This way it's safe.foriteminform.getlist("item"):do_something(item)

Parse input of type multipart/form-data (for file uploads).
Arguments are fp for the input file and pdict for a dictionary containing
other parameters in the Content-Type header.

Returns a dictionary just like urllib.parse.parse_qs() keys are the field names, each
value is a list of values for that field. This is easy to use but not much good
if you are expecting megabytes to be uploaded — in that case, use the
FieldStorage class instead which is much more flexible.

Note that this does not parse nested multipart parts — use
FieldStorage for that.

Convert the characters '&', '<' and '>' in string s to HTML-safe
sequences. Use this if you need to display text that might contain such
characters in HTML. If the optional flag quote is true, the quotation mark
character (") is also translated; this helps for inclusion in an HTML
attribute value delimited by double quotes, as in <ahref="...">. Note
that single quotes are never translated.

Deprecated since version 3.2: This function is unsafe because quote is false by default, and therefore
deprecated. Use html.escape() instead.

There’s one important rule: if you invoke an external program (via the
os.system() or os.popen() functions. or others with similar
functionality), make very sure you don’t pass arbitrary strings received from
the client to the shell. This is a well-known security hole whereby clever
hackers anywhere on the Web can exploit a gullible CGI script to invoke
arbitrary shell commands. Even parts of the URL or field names cannot be
trusted, since the request doesn’t have to come from your form!

To be on the safe side, if you must pass a string gotten from a form to a shell
command, you should make sure the string contains only alphanumeric characters,
dashes, underscores, and periods.

Read the documentation for your HTTP server and check with your local system
administrator to find the directory where CGI scripts should be installed;
usually this is in a directory cgi-bin in the server tree.

Make sure that your script is readable and executable by “others”; the Unix file
mode should be 0o755 octal (use chmod0755filename). Make sure that the
first line of the script contains #! starting in column 1 followed by the
pathname of the Python interpreter, for instance:

#!/usr/local/bin/python

Make sure the Python interpreter exists and is executable by “others”.

Make sure that any files your script needs to read or write are readable or
writable, respectively, by “others” — their mode should be 0o644 for
readable and 0o666 for writable. This is because, for security reasons, the
HTTP server executes your script as user “nobody”, without any special
privileges. It can only read (write, execute) files that everybody can read
(write, execute). The current directory at execution time is also different (it
is usually the server’s cgi-bin directory) and the set of environment variables
is also different from what you get when you log in. In particular, don’t count
on the shell’s search path for executables (PATH) or the Python module
search path (PYTHONPATH) to be set to anything interesting.

If you need to load modules from a directory which is not on Python’s default
module search path, you can change the path in your script, before importing
other modules. For example:

Unfortunately, a CGI script will generally not run when you try it from the
command line, and a script that works perfectly from the command line may fail
mysteriously when run from the server. There’s one reason why you should still
test your script from the command line: if it contains a syntax error, the
Python interpreter won’t execute it at all, and the HTTP server will most likely
send a cryptic error to the client.

Assuming your script has no syntax errors, yet it does not work, you have no
choice but to read the next section.

First of all, check for trivial installation errors — reading the section
above on installing your CGI script carefully can save you a lot of time. If
you wonder whether you have understood the installation procedure correctly, try
installing a copy of this module file (cgi.py) as a CGI script. When
invoked as a script, the file will dump its environment and the contents of the
form in HTML form. Give it the right mode etc, and send it a request. If it’s
installed in the standard cgi-bin directory, it should be possible to
send it a request by entering a URL into your browser of the form:

http://yourhostname/cgi-bin/cgi.py?name=Joe+Blow&addr=At+Home

If this gives an error of type 404, the server cannot find the script – perhaps
you need to install it in a different directory. If it gives another error,
there’s an installation problem that you should fix before trying to go any
further. If you get a nicely formatted listing of the environment and form
content (in this example, the fields should be listed as “addr” with value “At
Home” and “name” with value “Joe Blow”), the cgi.py script has been
installed correctly. If you follow the same procedure for your own script, you
should now be able to debug it.

The next step could be to call the cgi module’s test() function
from your script: replace its main code with the single statement

cgi.test()

This should produce the same results as those gotten from installing the
cgi.py file itself.

When an ordinary Python script raises an unhandled exception (for whatever
reason: of a typo in a module name, a file that can’t be opened, etc.), the
Python interpreter prints a nice traceback and exits. While the Python
interpreter will still do this when your CGI script raises an exception, most
likely the traceback will end up in one of the HTTP server’s log files, or be
discarded altogether.

Fortunately, once you have managed to get your script to execute some code,
you can easily send tracebacks to the Web browser using the cgitb module.
If you haven’t done so already, just add the lines:

importcgitbcgitb.enable()

to the top of your script. Then try running it again; when a problem occurs,
you should see a detailed report that will likely make apparent the cause of the
crash.

If you suspect that there may be a problem in importing the cgitb module,
you can use an even more robust approach (which only uses built-in modules):

This relies on the Python interpreter to print the traceback. The content type
of the output is set to plain text, which disables all HTML processing. If your
script works, the raw HTML will be displayed by your client. If it raises an
exception, most likely after the first two lines have been printed, a traceback
will be displayed. Because no HTML interpretation is going on, the traceback
will be readable.

Most HTTP servers buffer the output from CGI scripts until the script is
completed. This means that it is not possible to display a progress report on
the client’s display while the script is running.

Check the installation instructions above.

Check the HTTP server’s log files. (tail-flogfile in a separate window
may be useful!)

Always check a script for syntax errors first, by doing something like
pythonscript.py.

If your script does not have any syntax errors, try adding importcgitb;cgitb.enable() to the top of the script.

When invoking external programs, make sure they can be found. Usually, this
means using absolute path names — PATH is usually not set to a very
useful value in a CGI script.

When reading or writing external files, make sure they can be read or written
by the userid under which your CGI script will be running: this is typically the
userid under which the web server is running, or some explicitly specified
userid for a web server’s suexec feature.

Don’t try to give a CGI script a set-uid mode. This doesn’t work on most
systems, and is a security liability as well.

Note that some recent versions of the HTML specification do state what
order the field values should be supplied in, but knowing whether a request
was received from a conforming browser, or even from a browser at all, is
tedious and error-prone.